1. Field of the Invention
This invention relates to determining the concentration of oxygen in a gaseous atmosphere.
2. Prior Art
U.S. Pat. Nos. 3,907,657 to Heijne and 3,514,377 to Spacil et al relate to the measurement of oxygen O.sub.2 concentrations using solid electrochemical devices. For applications at elevated temperatures (&gt;500.degree. C.), for example, as might be encountered in the exhaust gases of furnaces or automobiles, the active material in these devices may be ceramic zirconium dioxide suitably adapted for the conduction of O.sup.= ions. Electrochemical cells made from this material are suitable at elevated temperature for oxygen sensing and pumping applications.
The mode of operation of the Heijne device can be described as an oxygen counting mode in which oxygen partial pressure is determined on a sampling basis. A constant current (or equivalent means) is applied to an electrochemical cell which forms part of the enclosure of a volume for a period of time, t.sub.p for the purpose of electrochemically pumping out most of the oxygen from that volume. The ambient atmosphere had established itself within the volume prior to the pump out by means of a leak. An additional electrochemical cell, which serves as a sensor of the reduced oxygen partial pressure within the volume and which also constitutes a portion of the enclosure, provides a signal indicating when oxygen has been sufficiently depleted from the volume (see FIG. 4 of Heijne). Knowing the temperature, enclosed volume and the pump out current and time allows one to calculate the number of oxygen molecules within the enclosure from the ideal gas law. The number of oxygen molecules is in turn proportional to the desired oxygen partial pressure. If a constant pump current is used, the pump-out time t.sub.p is proportional to the oxygen partial pressure. If a constant current is not used, then the integral of the pump-out current over the pump-out time is proportional to the oxygen partial pressure.
The Heijne device can provide an output which is linearly proportional to the oxygen partial pressure. This is superior, for example to single oxygen concentration cells used as sensors which give an output (EMF) proportional to the natural logarithm of the oxygen partial pressure ln (P.sub.O.sbsb.2).
A potential disadvantage of the Heijne device is response time. For this measurement procedure, the leak connecting the ambient to the enclosed volume must be small so that during the pump out of oxygen, no significant amount of oxygen leaks into the volume to cause an error in the count of molecules (i.e., to erroneously increase t.sub.p). However, if the leak is made small, it may take a long time, for the ambient to reestablish itself within the volume after a pump out. If the changes in the oxygen partial pressure in the ambient occur rapidly with respect to this time, then the device would not be able to follow these changes with repetitive operation.
U.S. Pat. Nos. 3,923,624 to Beckmans et al, 3,654,112 to Beckmans et al, and 3,907,657 to Heijne et al describe tubular ceramic structures for measuring and controlling the composition of oxygen in a carrier gas. In some cases a pump cell and a sensor cell are used. U.S. Pat. Nos. 3,923,624 and 3,654,112 teach devices to be used primarily to dose a gas with oxygen to a constant partial pressure. Measurement of the dosed gas is made by a standard technique using a zirconium dioxide oxygen concentration cell to be sure that the dosed gas contains the correct amount of oxygen. The sensitivity of the concentration cell to the oxygen partial pressure is low, being proportional to the natural logarithm of the oxygen partial pressure. This purpose is divergent from the purpose of measuring with high sensitivity the oxygen partial pressure in a feedgas. There is no suggested application of these devices for an auto exhaust.
In the case of the teachings of U.S. Pat. No. 3,698,384 to Jones, the purpose is to measure oxygen partial pressure in a feedgas. This is done by measuring the pumping current while holding the sensor cell voltage a constant. However, to achieve a result in the disclosed open ended tubular structure made from zirconium dioxide, the flow rate of the feedgas must be kept constant. If the flow rate should attempt to vary, there is a relatively elaborate flow control circuit to keep the flow rate constant. This scheme, which also employs a reference atmosphere, is relatively unsuitable for application in an auto exhaust where the exhaust flow rate would change substantially with RPM.
U.S. Pat. Nos. 3,347,635 to McKee and 3,857,771 to Sternberg both describe oxygen sensing procedures or devices wherein the taking of a first derivative of an output signal either determines the oxygen partial pressure or can yield information about the medium which contains the oxygen. Neither device would be suitable for the continuous or repeated determination of the oxygen partial pressure in a variable, high temperature environment like that occurring in an automotive exhaust.
FIGS. 1 and 2 of the drawings illustrate a known oxygen pumping sensor in which ionically conducting zirconium dioxide with thin platinum electrodes 2 and 3 form an electrochemical pump cell which with additional ceramic structure 4 define an enclosed volume 6. The ambient atmosphere can establish itself within the volume by means of a leak opening 5. A battery 7 is attached to the electrodes by means of lead wires 8 and 8'. A voltmeter 10 and ammeter 9 are provided to determine the voltage drop across the pump cell and the current flowing through it. Although similar in structure to FIG. 5 of U.S. Pat. No. 3,907,657, the operation is different. Here one applies a pump voltage V to remove oxygen from an enclosed volume 6 until the pump current saturates. The saturated current is proportional to oxygen concentration. This saturation property is shown in FIGS. 3 and 4.
This is a steady-state device. When steady state is reached, the flow of oxygen through a leak opening 5 equals the pump current times a proportionality constant. The current saturates at a voltage greater than about 0.5 V because the leak in combination with the platinum electrode 2, the cathode, will only allow a limited (saturated) amount of oxygen to enter and be electrochemically pumped from the volume per unit time. The device has the advantage of giving an output signal (the value of the limiting current) which is linearly proportional to the desired ambient oxygen partial pressure. However, to the extent that the saturated current value depends on the detailed properties of electrode 2, the device calibration may be subject to drift as these detailed properties may change during the sintering and wear of this thin layer.
An important application of high temperature oxygen sensors is in the determination of the stoichiometric air-fuel mixture in the exhaust gases of hydrocarbon fired furnaces or engines such as automobile internal combustion engines. The stoichiometric mixture is one in which the mass of air pressure contains just enough oxygen to react with the mass of hydrocarbons present so that there is the minimum amount of both oxygen and hydrocarbons remaining. For common automotive gasoline, the air fuel ratio (A/F=mass of air/mass of fuel) at the stoichiometric point is approximately 14.6. If, for example, an engine were running lean of stoichiometry (e.g., an air-fuel ratio greater than 14.6, there would be an excess of air in the "charge" and the exhaust gas would contain a substantial oxygen partial pressure. If rich operation were occurring, e.g., an air-fuel ratio less than 14.6, the exhaust gas would contain unreacted or partially reacted hydrocarbons and very low oxygen partial pressure. In particular, the equilibrium oxygen partial pressure in the exhaust gas can change by a great amount (as much as 20 orders of magnitude) as one moves from lean to rich operation. This large change forms the basis for detecting the stoichiometric air-fuel ratio with an exhaust gas oxygen sensor. The electrical output of such a sensor can then be fed back to an electrically controllable carburetor or fuel injection system for maintaining engine operation always at the stoichiometric point. Depending on engine type, operation at this point frequently offers a reasonable compromise for minimizing regulated exhaust gas emissions and maximizing engine performance.
There are known high temperature oxygen sensors utilizing oxygen electrochemical concentration cells (usually made from zirconium oxide) and requiring the use of a reference atmosphere (usually air) which are suitable for determining the stoichiometric air-fuel ratio. These devices give an output (EMF) proportional to natural logarithm of the oxygen partial pressure. Despite their low sensitivity to oxygen partial pressure, the large change in oxygen partial pressure at the stoichiometric point allows their useful implementation.
For some engines it is useful to operate lean of the stoichiometric A/F for the purpose of reducing fuel consumption. Oxygen partial pressure varies in a systematic way in the lean region and this can form the basis for determining lean A/F. The exact knowledge of lean A/F would be useful to fully implement a lean burn engine strategy which would maximize fuel economy and engine performance and minimize regulated emissions. However, the variation in oxygen partial pressure in the appropriate lean A/F region, e.g., air-fuel ratios greater than 16, is not large, (in comparison to the changes occurring near stoichiometry) so that suitable oxygen sensors with sensitivities greater than the natural logarithm of the oxygen partial pressure are desirable for accurate measurement in the desired A/F range. These are some of the problems this invention overcomes.